Re: [PATCH v3 3/7] trace-cmd: Fixed logic in tracecmd_read_page_record()

[Steven Rostedt <rostedt@goodmis.org>]

On Fri,  8 Feb 2019 15:49:14 +0200
Tzvetomir Stoyanov <tstoyanov@vmware.com> wrote:

> Fixed the logic in tracecmd_read_page_record() when
> searching for the next event in the event from the
> ring buffer. In the current logic, in the case when
> last_record is not NULL (we are searching for the second one)
> the check "if (ptr != last_record->data)" always fails.
> 

Nice catch.

Let's change the change log and subject:

Subject trace-cmd: Fix tracecmd_read_page_record() to read more than one event

 The kbuffer_next_event() will return the next event on the sub buffer.
 If we pass in the last_record to tracecmd_read_page_record(), it
 initializes the sub buffer, and by calling kbuffer_next_event()
 (instead of kbuffer_read_event()), the second event on the sub buffer
 is returned. This causes the match of the last_record not to match if
 the last_record happens to be the first event on the sub buffer.


> Signed-off-by: Tzvetomir Stoyanov <tstoyanov@vmware.com>
> ---
>  lib/trace-cmd/trace-input.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/trace-cmd/trace-input.c b/lib/trace-cmd/trace-input.c
> index 33ad7d1..5d64ca8 100644
> --- a/lib/trace-cmd/trace-input.c
> +++ b/lib/trace-cmd/trace-input.c
> @@ -1688,18 +1688,22 @@ tracecmd_read_page_record(struct tep_handle *pevent, void *page, int size,
>  			goto out_free;
>  		}
>  
> -		do {
> +		ptr = kbuffer_read_event(kbuf, &ts);
> +		while (ptr < last_record->data) {
>  			ptr = kbuffer_next_event(kbuf, NULL);
>  			if (!ptr)
>  				break;
> -		} while (ptr < last_record->data);
> +			if (ptr == last_record->data)
> +				break;
> +		}
>  		if (ptr != last_record->data) {
>  			warning("tracecmd_read_page_record: could not find last_record");
>  			goto out_free;
>  		}
> -	}
> +		ptr = kbuffer_next_event(kbuf, &ts);
> +	} else
> +		ptr = kbuffer_read_event(kbuf, &ts);
>  
> -	ptr = kbuffer_read_event(kbuf, &ts);

Also, instead of the above "} else" clause, we can just move the

	ptr = kbuffer_read_event(kbuf, &ts);

before the if condition testing last_record.

-- Steve

>  	if (!ptr)
>  		goto out_free;
>